TRAVERSE CITY,
Mich. (AP) -- Invasive grass carp have reached three of the Great Lakes and
pose a significant environmental risk there, but time remains to prevent
them from getting out of hand, according to a scientific analysis released
Friday.

The voracious grass
carp is among four Asian carp species threatening to reach the world’s
largest surface freshwater system. Bighead and silver carp, the most feared,
would compete with native fish that eat microscopic plants and animals,
while grass carp feast on aquatic vegetation that provides crucial habitat
and spawning grounds.

Grass carp have
been found in Lakes Erie, Michigan and Ontario, although it’s uncertain how
many there are or how widely they have spread, U.S. and Canadian researchers
said. At least some are reproducing.

“For the first
time, we have a binational, peer-reviewed study by some of the best minds
and practitioners in the field who have a consensus on what the risk is to
the Great Lakes from grass carp, and it’s pretty substantial,” said Marc
Gaden, spokesman for the Great Lakes Fishery Commission.

Grass carp were
introduced to the U.S. in the early 1960s to control weed growth in
waterways. Like other Asian carp, some escaped into the Mississippi River
and have migrated northward toward the Great Lakes.

It has long been
known that at least a small number of grass carp were in the lakes, Gaden
said. Some may have slipped into Lake Michigan through a Chicago-area
waterway network before electric barriers were erected to block fish
migration. People might have released others, intentionally or by accident.

“They’ve just been
humming in the background,” Gaden said. “They haven’t gotten a lot of
attention. Once in a while one would get captured.”

But they have
turned up more often in recent years and the threat of a full-fledged
invasion appears to be rising, he said. A particularly ominous sign is that
some of those caught have been fertile. Grass carp reared in hatcheries,
which some states allow, are supposed to be sterilized before being released
into the wild.

Twenty-three grass
carp have been caught in Canada since 2012, including five in Lake Ontario
at Toronto, said Becky Cudmore, Asian carp program manager for Canada’s
fisheries and oceans agency and the report’s primary author.

“Right now, the
sterile fish outnumber the fertile fish,” Cudmore said. “This isn’t game
over, but we are finding more of these fertile fish.”

The analysis said
it is “very likely” that grass carp will become established in Lakes Erie,
Huron, Michigan and Ontario within 10 years unless effective steps are taken
to stop them. The risk is lower in frigid Lake Superior, which offers less
food.

An established
population is one that reproduces over multiple generations.

Researchers are
studying how to prevent that, Gaden said. Tougher enforcement of laws
against bringing them into the region would help, along with greater care to
prevent release of fertile fish from hatcheries.

Another possibility
would be using nets to block their path to spawning areas during times when
they reproduce, Cudmore said. Asian carp are known to spawn later than
native fish.

“Our assessment is
saying that yes, they were showing up before, but now they’re starting the
invasion process,” she said. “They have arrived. Now is the time to act.”